Only small tanks of 10 gallon or less are no more than 20 inches long.
I have no idea what "smiling fish" is. Never heard of it. What is the name of the species?
As for clownfish, there are more than 30 different clownfish species. Which one do you want? The smallest clownfish specie will be close to 4" long when full grown, and the biggest will be more than 7" in length. You can't have any of them in a 10 gallon. They need a bigger tank.
If you can allow longer length for the tank, lets say 24"... you can get a 20 gallon tank. 30" long will give you up to 29 gallon.
If the tank size can't be upgraded, you should stay with fish species smaller than 3" long. For a 10 gallon, you may have up to 5~6 fish of around 3" long before it is fully stocked. Any more fish will overload the filtration and create ammonia spike.
